A case history exchange was carried out to study the diagnosis of hyperkinesis (Attention Deficit Disorder) and Conduct Disorder in Europe and the United States. Both research teams at the NIMH and the Maudsley Hospital in London, as well as clinician teams in Washington, D.C. and London, rated 40 case histories, 20 with videotapes. Both the International Classification of Disease - 9th edition (ICD-9) and Diagnostic and Statistical Manual (DSM III) were used by all raters. Preliminary data analysis shows that much of the wide discrepancy between European and U.S. rates is due to the different schemes involved. ICD-9 only permits one diagnosis and Conduct Disorder is so defined as to be quite inclusive. Thus, the research teams agreed well within schemes, both diagnosis Conduct Disorder more frequently using DSM III which permits multiple diagnoses. Agreement was good between research teams within each scheme. Preliminary analyses of the clinician team data shows both influence of diagnostic schemes as well as an influence of the nationality of the clinician. While still diagnosing Attention Deficit Disorder more frequently using DSM III, British clinicians also implement the schemes differently.